Allows to "pass in a logger" rather than using the customizable String->Logger func

[ktoso]

Motivation:

• Developers may have very opinionated and "not global" opinions about loggers
  • I recently talked with a team which might consider wanting to put all logs from the actor system (and things it invokes) into a separate log file, separate from access logs of a vapor app
  • this is doable today, but a bit "un-usual" by overriding the system.settings.logging.overrideLoggerFactory so we've always thought about this, but other teams really like to "pass in a logger" which here becomes a bit weird.

This allows the following patterns, while keeping all our functionality about LogCapture and similar.
Tons of discussions with @weissi led to this current rehash of how we create loggers.

We do not inherently depend on the source addition, but there are some spots where it would be the right thing:

• for serialization
• maybe for dead letters
• for the top level "system logger"

We would not use "source" for actor paths through as those we do want to "inherit" when we'd call some other library, so it has to be a Logger and not a LoggerWithSource.

This rehash allows for the following pattern:

let rootLogger = Logger( ... my special logger, maybe even custom handler)
ActorSystem(...) { settings in 
settings.logging.logger = rootLogger

This logger is used as "template" to create new loggers in the entire system; an actor's logger is a copy of this logger + its actor specific (e.g. path) metadata set when the actor is created.

Modifications:

Removes

//    /// Optionally override Logger that shall be offered to actors and the system.
//    /// This is used instead of globally configured `Logging.Logger()` factories by the actor system.
//    public var overrideLoggerFactory: ((String) -> Logger)?

In favor of

    public var logger: Logger = LoggingSettings.makeDefaultLogger()

    static func makeDefaultLogger() -> Logger {
        Logger(label: "<<ActorSystem>>")
    }

which users may change.

Result:

• More flexible logging configuration.
• "label" is a bit useless, we can't control it
• actor paths are ONLY present in actor/path metadata, nowhere else (they used to be in the label as well, but that was slightly wrong semantically)

[ktoso]

maybe... not sure tbh.

When using the "globally set up one" this is useful since one can just set the level easily.

[ktoso]

this is replaced by allowing to pass in the "base logger"

[ktoso]

provides a nicer printout in our metadata aware LogCapture

[ktoso]

small improvement while I was in here, easier to look at nested metadatas now in the LogCapture

[ktoso]

that's the "main" change I guess; we no longer do the override factory but pass in a logger than can be a specific one.

FYI @weissi

[ktoso]

FYI @budde, with this change it'll be possible to directly "set" the "base" logger on an actor system.
It was previously already possible via setting overrideLoggerFactory if you wanted to have a logger specific to the actor system i.e. "some other file" like we discussed. With this change this is a bit simpler.